Stool specimens from 90% of the population of 2 marginal communities in Coatzacoalcos, Mexico, were examined by the Kato-Katz technique for the estimation of eggs per gram of faeces (epg). Average epg values for Ascaris lumbricoides and Trichuris trichiura were calculated for each family in the community, based on the arithmetic mean epg of family members for each parasite. 39 families were classified as either lightly or heavily infected with A. lumbricoides and infected members were treated with mebendazole. A. lumbricoides expelled following treatment were collected from the children 2–10 years of age, and reinfection was monitored monthly for 6 months. At the end of the reinfection interval, a final stool specimen was collected from all family members and those infected were treated. Again, A. lumbricoides expelled by the children 2–10 years of age were collected. There was a significant correlation between the numbers of A. lumbricoides expelled by the children at the first and second treatments as well as in epg values before the first and second treatments for both A. lumbricoides and T. trichiura in children (2–10 years) and adults (>19 years). At the family level, there were significant correlations between the values of family mean epg before each treatment for both A. lumbricoides and T. trichiura.